Linux kernel versions 6.6.102-104 and 6.12.42-45 have a regression
in /proc/net/tcp that causes seek operations to fail with "illegal seek".
This breaks portlist tests on these kernels.
Add kernel version detection for Linux systems and a SkipOnKernelVersions
helper to tstest. Use it to skip affected portlist tests on the broken
kernel versions.
Thanks to philiptaron for the list of kernels with the issue and fix.
Updates #16966
Signed-off-by: Andrew Dunham <andrew@tailscale.com>
